Electron gun structure



Feb- 6, 19541 R. E. JOHNSON 2,540,621

ELECTRONI GUN STRUCTURE Filed Feb. 19, 1948 INVENTOR azz E. Jolmfazz ATTO R N EY Patented Feb. 6, 1951 ELECTRON GUN STRUCTURE Ralphy E. Johnson, Lancaster, Pa., assignor to Radio Corporation of America, a. corporation of Delaware Application February 19, 1948; Serial No. 9,420

'7 Claims. 1-

This invention relates to television pickup tubes andv` in particular'to a novel electron beam generator gun structurefor use in such tubes.

.One type. of television pickup tube uses a low velocity electron beam'. which is scanned across a target electrode to discharge or neutralize a chargespattern comprising adistribution of positive electrostatic; charges on the target surface. If velocities of emission and contact potential are neglected, the equilibrium of the. target is d'eterminedbythe potential of the electron gun cathode electrode. The current in the beam is determined. by that required to'completely'dischargeV the most positive areas'of the-target. The scanning electron beamapproaches the target at nearly zero velocity. The negative beam will strikeand discharge-the positively charged areas of. the.V targetand charge the target areas negatively until thef surplus portion of the beam is repelled back along the tube axis. Thus, an electron beam of uniform density scans the target while a non-uniform or modulated beam is returned Vtothecathode endof the tube Where it is collected and amplified as-the. output signal.

The spot formed by the intersection of the scanning electron beam With the target must@ not onlyv besmall but the scanning beam .must be one in which the velocity component of the electrons normal to the target is as nearly uniform as possible. This isequiva'lent, if spurious potential gradients along the target surface to to be avoided, to requiring the angle of incidenceof the beam tov bisconstant over the target surface. At normal incidence, a given beam current' will charge -artarget area morefnegatively before it is repelled than when the beam. approaches the target at other angles I of incidence.

The beam is unable/'to discharge certain Vareas completely when the incidence angle isbtoo far from the normaLrleaving these areas positively charged while other'areas arenot completely discharged. The inability of the beam'to landin areas where it should land produces shading. The loss of picture contrast'impliesa loss' of 'detail. When the beam landing is impaired'f the sharpness of the picture is impaired. This would result fromv a selective landing' oftheV4 beam at points on the target ofrelatively high potential. That is, there; isV a lossof 'truevidemsignal'and consequently disruption of resolution due to selective landing of the beam on target areas lof higher potential.

A conventional. electron gun structureV comprises a: positively charged accelerator electrode Y and spacedv from an electron emitting cathode surface. Positioned between the accelerating electrode andthe cathode is a negatively .biased control electrode. A gun of this type produces aA largeV range of electron velocities. The coinponents of the electron beam having low axial velocities are inflarge part the electrons coming fromthe outer circumferential portions of the cathode` surface. The electron lens formed by the cathode and the negative gridin a conventional gun structuredraws on or uses these low velocity components of `the electron beam. The large electron velocity range inherent in this construction Will producea beam,` in which the components strike the target atvarious angles of incidence to produce the above. described spurousshading in the transmittedpicture. It is desirable therefore. that television. pickupl tubes utilizing a lowvelocity scanning beam be operated with. a beam having the smallest possible range of electron velocities.

It is` therefore an objectof my inventiont to provide an improved television `pickup tube.

It is-a furtherl object of-my invention to provide `animproved electron gunstructure forV use in a television pickup tube.

Itis, another objeotof my invention t0 provide an improved electron gun .structure Vfor producing a cathode ray beamwitha small range of electron velocities.

It is, furthermore, an object of my invention toV provide an improved .television pickup tube in which the target is scanned at all points by. an electron beam, a large partlof which has the same componentof velocity normal to the target.

It is also an object of the invention to have .the component` of the velocity normal to the target represent a very large part of the total velocity in the. beam.

The novel features which I believe to be characteristic ofmy invention are set forth with particularity in the appended claims, but the invention itself will best be undertsood by reference to the. following description taken in connection With'the accompanying drawing, in which:

Figure lis a sectional view-of a television pickup tubemade according t0. myinvention; and

Figure `2r is a'sectional view of an electron gun assembly used in the television pickup tube shown of- Figure-1.

Figure 1 disclosesa television pickuptubefor producing; a video signal for transmission. The tube comprises an envelope I 'preferably of glass. Atone end ofthe tube isfpcsitioneda photocathode electrode l4,formed as a Apbotosensitive coating on the inner surface of a transparent face plate I2, closing the end of the tube envelope I0. This Photocathode I may be of any conventional type well known in the art. Positioned ad- `iacent to the photocathode I is an accelerating electrode IG and axially spaced therefrom is a cup shaped electrode 2| for supporting a glass target electrode I8. Between the target electrode I8 and the photocathode |13, is a very fine screen mesh 20, closely spaced from the surface of the glass target I8, facing the photocathode Ill and conductively supported by the cup-shaped electrode 2|. This much of the tube forms the image section of the television pickup tube.

An object I5, which is to be televised, is focused by an appropriate optical system, represented at |I, in Figure 1, as an optical image I9 on the photocathode electrode I4. The photocathode It and the accelerating electrode IB are preferably biased, respectively, at approximately a negative 400 volts and a negative 320 volts relative to the potential of the support electrode 2| which in turn is maintained at close to 2 volts positive relative to a reference or ground potential. This arrangement produces an accelerating field between photocathode I4 and the glass target I8. A focusing coil 28 surrounds the envelope of the tube, as shown, and forms a uniform focusing field whose lines of force are parallel to the axis of the tube.

The optical picture I9 focused upon the sensitized photocathode surface Iii releases electrons in quantities relative to the intensity of the incident light. These electrons pass through the accelerating field of electrode IS and strike at high velocity the surface of the glass target I8. The magnetic focusing field of coil 28 directs the photoelectrons from electrode I4 along straight paths to the glass target I8. The velocities at which the photoelectrons strike the glass surface I8 cause a secondary emission greater than unity of electrons from the glass surface. The fine mesh screen acts as a collector electrode of the secondaries emitted from the glass surface I8.

The target I8 is a thin sheet of low resistivity glass. The emission of secondary electrons from the surface of the glass target I8 leaves areas of the surface of the target struck by the photoelectrons positively charged. The insulation properties of the glass target are such that there is little or no electrical conduction along the glass surface. Consequently, there is formed on the surface of the target facing the photocathode |13 a positive potential distribution corresponding to the distribution of light and shadow of the optical image IS focused upon the photocathode I.

The opposite side of the glass target I8 is scanned, during tube operation, by a low velocity electron beam 22 formed by an electron gun structure 2li mounted at an opposite end of the tubular envelope I0. The cathode 50 of the electron gun structure 2li, shown in detail in Figure 2, is maintained at the reference or ground po.- tential referred to above. The electron beam 22 is constrained by the axial field of coil 28 to follow a path along the axis of the tube when no deflecting fields are applied. Conductive electrode coating 26 on the inner surface of the tube envelope is maintained at around 200 volts positive, relative to the reference potential, to sustain within the enclosed area a uniform electrostatic field. A decelerating electrode 32, adjacent the glass target I8, is maintained during tube operation at around 75 volts positive potential, relative to the reference potential, for maintaining a 4 decelerating field to slow down the electrons of beam 22 so that they approach the glass target at close to zero velocity.

The beam 22 is caused to scan the surface of target I8 by 2 pairs of magnetic deflection coils on axes normal to each other. The elds of the coils 3E! are perpendicular to each other and to the axis of the tube IB. It will be understood that the defiection coils 30 will have periodically varying voltages applied thereto, for example, by sawtooth generators (not shown) of suitable frequencies, to produce line and frame scansion. Due to possible misalignment of the gun structure 24 relative to the tube axis, an alignment coil d6 is provided to maintain a small magnetic field perpendicular to the axis of the electron beam. Rotation of the coil 46- around the stem of the tube envelope Iii will tend to correct any misalignment of the electron beam relative to the axis of the tube as it leaves the electron gun 24.

As beam 22 scans across the face of the glass target I8, sufficient electrons land to maintain the scanned target surface at cathode or reference potential. The remainder of the beam approaching the target at close to zero velocity is reflected by the target surface to form a return beam 25. This return beam 25 will pass back along the same path as the incident beam 22 and will strike a large intercepting ldynode surface 66 forming the first stage of a multiplier unit described in detail in Patent No. 2,443,941, issued January 6, i948, to Paul K. Weirner. The dynode surface 60 is maintained at 300 volts positive relative to target 28 and is also sensitized to produce amplification of the return beam 25 by a secondary emission greater than unity. An electrode 3:1 which is also maintained at around 290 volts positive forms a field free space around the dynode surface 66 so that the secondaries emitted from the sensitized surface are not drawn with the primary scanning beam to the target but are persuaded to pass into an accelerating field of a second dynode electrode 36 maintained at 600 volts positive relative to target E8. A third, fourth and fifth dynode of the multiplier unit are provided respectively at 38, 40 and 42, so as to amplify the return beam in successive stages. Each dynode, respectively, is an apertured electrode of successively higher positive potential and having sensitized or electron emitting surfaces so that the electrons strike each dynode at sufficient velocity to set up a secondary emission greater than unity. Multiplier electrodes 38, 40 and 42 may be respectively maintained during tube operation at 880 volts positive, 1160 volts positive and 1450 volts positive relative to reference or ground potential. A collector screen electrode 44 may be maintained at around 1500 volts positive.

While the wall coating 26 and the various electrodes and dynodes associated with the tube, as described above, may have various desired potentials, the potentials respectively mentioned for each electrode represent those used in a successfully operated tube of the disclosed type. As shown in Figure 1, the electrodes may be connected to appropriate points of a potentiometer or bleeder resistance R, connected at its ends to a source of direct current potential.

In the operation of the tube of Figure 1, the scene I5 to be transmitted is focused on the transparent photocathode I. Photoelectrons are released in direct proportion to the brightness of the various parts of the scene. The photoelectrons strike the target with suicient velo'cty ltocause a .seeondaryemission ratiocgreatenthan unity as. describedabove form a positive charge. pattern` on. the. target t8; the more positive- .areas ycorresponding to the highlights of.I the scene. 'IP-he .glwsoi-targ-et is fsufiicientlytln'n lthat ieach positive .area on. the4 photocathode side. will i .oil i target t8, thebeam Yof' electrons '2a is. scanned acrossfthevop'posite sideol the targetanddeposits .suicient electronsto neutralize-the positiverarea .ofrthertarget to` :the reference :or cathode. poten.- tialf. "IT-heglasstarget mi .is .chosen withwa resistivity low. enough .that electrostatic .charges` deposited on-opposite.sideso the glassare. neutral `ized.A by conduction. through. `the. glass duri-ng .a irirametimeof around-'lo .of asecond.. rhetarget iis also. thin enough so thatthese charges-do not spread laterally, in arframe time, suilicienitlyv to impair-:the resolution `o'fthecharge pattern-.

As-aspecie positivearea otthetargetsurface, struck by. the. scanning beam 25, is returned.l to `reference orground potential, it will reflect the excess portier-if of-1the scanning beam to for-m` a irnodulatedielectron. return beam .ggwhichhas a varying .density determined by the. chargefpab tern of. target'. i8. The returni-ng electrons of beam will-closely -iollowthe lines-oi them-agnetic focusing eldformed-bycoil E-flcack toward the gunrZli-y and.I strike dynode surface 55 to generate. a larger number-ofisecondary electrons. 'Theseveralstages or the electron multiplier, as :described` above, amplify thlismodulated return beam-rato formthe video signal, which istaken .oft-ot the collector- Figur-rc2 shows` detail. the structure of an electron-gun, according to myinvention, for-usein .a -televis'ion pickup tube. ci the Vtype described ia'bovei A tubularoathode .electrode 55: isr -co- '.axiallyJmou-ntedvvithin a .supporting-:cylinder 54;

'The mounting. means .comprises a ceramic support. member zfziixed `to the cathode -tube '5 ll and litted within .thessupport cylinder-51land ar ranged.; `as..disclosed.infFigure 2,- toA spacca-closed .endilof 'cathodeltube 5-'from anapertured con.- trol grid disc 56 with an opening 5-1', axially aligned.` with. the "tubularv cathode 58. Also, mounted with. .the cylinder 5e, is. a secondA apertured electrodev Aii:axially.spaced= within 'the supportingtube 155i. by the ceramic rin-g.V 58; 'The apertured. .electrode E il is spaced.:- by -a- .metal ring 62.. from Va` partially closed.A end-o.- the. supporting cylinder 54. The several electrode parts, described: above, are. mountedr'within the support cylinder 514! by .successively -insertingthemwithin wthesupport cylinder., starting with the metal :spacing ring. :62. These. parts then. arelocked within supporting cylinder ..511 .byfa vretainer .ring -'iwelded, .or "fired zbyeany Viother-..means, to. the inner. Isuriacesoff cylinderid'. Mounted `Within the .cathode :tube` 5t -.is f.a.lieater..ilamentil` which extends, asis shown, from .anopen..'.end-oi the -cathode tubes. 'Heaterfilamentf Ms supported from. .avsupporti-ng ceramic disc 32; locked: inposi- :tion` .against the ring, "iii, by a second; retainer 'ringr-:welde-d, or fixed, vto.theinner-surface.of .supporting-.tube 56.- A thirdiaperturedfelectrode Semis' mountedon the partiallyfclosedendof the .support cylinder il. Thisielectrode ifcomprises anlaperturedvsupporting cup lil .having an raper- -turedLsupport plate 6 Sfeoaxially xed to .the .bot- :itomL surface. thereof; as is.:shown Figure-2. "Coveringthesupporting plate Eisa thinliigmy polished. .metal sheet 65; Ypreilerably-ot' a silver magnesium alloy, which forms the. secondary emissir'e surface for the rst dynode. electrode. Through the center of the. plate. 5S along the axis of the gun is formed a very small aperture Sie whichprovidesh masking for the electron beam of the gun .24; The mask-ing aperture. 55 limits the crossfsectional area o the electron. beam to a small centercore portion which passes through aperture 65. as thev beam 22.

'Electrode is preferably inaintainedfat a negative potential, relative to cathode potential, to provide a control for .the electron emission from the cathode. 5t. The closed .end 'i3 ci the cathode tube 5'@ is covered with` anv activated materialize providea source ciy electron emission. The material may be a mixture of.` the oxides ci barium and. strontium. and may be applied to cathode surface i8, .as is Well. known in the rrt. During tube operation, heater filament Sii. maintains the activated cathode surface "38. at a temperature sufficient to provide for an. emission of electrons. The cathode is maintained.- at reference potential, during tube operation and electrode@ is preferably maintained at around 3G() volts positive, relative to the cathode electrode. This electrode idraws the electron emission from 'the `cathode surf-ace 718'; forms it into a beam 22. accelerates it to high velocity before the crossover possible.

pointof the beam at 23. Electrode iandmask ing electrode i3-fl aremaintained at thesame-.potential `through the .conductivity of the metallic spacer ring 62.

Ordinarily in television pickup tubes of the type described, the electrons oi the scanning beam emerge from the electron gun at high velocity and With transversevelocity components, whose energy may range fromV zero to aV volt or more. As each electron of the beam emerges into the magnetic focusing lield of the tube, it will describe a helix. Also, because `of the magnetic focusingeld, the beam Will converge to a focus at several nodal points between the gun and target. Those electrons With the greatest transverse .component of velocity will spiral to the greatest distance iromthe .beam and will Aapproach the target at the greatest angles Voi incidence. The electrons which possess transverse energy have acquired this ener-gy at the lexpense .or theirlongitudinal-or axial energy and will thus also approach the target at a smaller normal velocity component. Hence, the electrons of the beam, which have acquired the .greatest transverse velocity component, will not be enabled to reach the target beforereiiection. Thus, a scanning beam of electrons having a large range of velocities will not discharge a positive area of thetarget as completely as one in which the electron velocity range is smaller and in which the velocity component normal to the target of the electrons is as nearly uniform as If the scanning beam fails to disrcharge eachv positive area .of the target completely, not only will there be produced on the target undesirable potential gradients, which Willresult. 1in-` spurious shading .of the video signal, but-there will'be a larger unused portion of. thereturnbeam, Which will result in poorer modulationof the .return beam and also .will con- .tributetoxthe noise outputof the multiplier without adding to the signal.

My specific electron gun structure improves `the operation of the 'type of television pickup tbefdescri-bedf by reducing Vthe range-ofelectron velocities in the-:scanning beam; Thel sections 'of the beam having low axial velocities consist of electrons coming from the outer circumferential areas of the cathode. The specific gun structure, shown in Figure 2, tends to eliminate these electrons having the greatest transverse velocity component and thus reduce the electron velocity-range of the beam. Control electrode 55 is closely spaced from the sensitized electron emitting surface 18 of the cathode 5G and is maintained at a constant negative bias of around a -70 volts, relative to cathode potential, during tube operation. In one example of the electron gun shown in Figure 2, the cathode to control electrode spacing is close to 0.0075 inch. To overcome the blocking effect produced by this large negative bias of grid 56 and its close spacing from the cathode, and also, to provide a strong positive field to draw suicient electrons from the center of the cathode, I have positioned the accelerating grid 60, at the high positive potential of 300 volts positive, close to grid 56. Accelerating grid 60 is closely spaced from electrode 56 by a distance of around 0.031 inch. The aperture 5l, in grid 55, has a diameter of around 0.045 inch, so that the effective emitting area of the cathode surface 18, which can see the large positive field of the accelerating electrode 60 through aperture 5l, is relatively small, and the beam, drawn from this small area at the center of the cathode emitting surface, has a narrow range of electron velocities.

This electrode arrangement results in several advantages. The presence of a control grid closely spaced from the cathode emitting surface and having a large negative bias suppresses emission of beam electrons from the outer circumferential area of surface 18 and which would have the lowest axial velocities. Also, the small eiective emitting surface of the cathode produces an electron beam having less dispersion as electrons in coming from a small cathode area follow more parallel paths. This results. in a beam of smaller cross-sectional area. The accelerating grid Sil, maintained at high positive potential, pulls a suiciently large beam current from this small emitting portion of the cathode surface and also accelerates the electrons to their greatest velocit-y, before their crossover at 23. This produces an eiective packing of the electrons, since in going through the crossover point at high velocity, there is less tendency of the beam to spread from mutuel electron repulsion. The masking aperture 65 selects only the core of the electron beam diverging from the cross-over point 23. is in the order of 0.002 inch and is carefully aligned with apertures Si and 5l, so that the center of the beam will pass through the masking electrode 6d. Furthermore, the small masking aperture 65 eliminates any diverging electrons from the outer areas of the eiective-cathode emitting surface and passes electro-ns having more uniform velocities. This selection of the beam core results in a scanningv beam having a smaller velocity range.

Another advantage of arrangement, of Figure 2, is the maintaining of a high potential in the region of the beam crossover point 23. It is well recognized, that a better focus of an electron beam is obtained the higher the potential of the field is, at the cross-over point. The spaced electrodes 50 and 64 are maintained at a common potential of 300 volts positive, relative to cathode or references potential. This is the highest accelerating-potential The masking aperture 65 the novel electrode *l imposed on the beam 22, between cathode 50 and target I 8. Electrode 60 is recessed to form a substantial region between it and the limiting electrode 64 in which cross-over 23 point is formed. The spacing of electrodes 60 and 64 in the tube described is not very critical and is in the order of 0.117 inch. The spacing between the accelerating electrode 60 and control electrode 56, as well as their relative potentials, control the crossover point 23. In the particular gun described, if accelerating electrode 60 is closer than 20 mils from electrode 56, the cross-over will extend into aperture 65 of masking electrode 64; This will result in a poor beam, as the effect of the limiting or masking aperture would be lost, and the beam would have wide divergence. Best results are obtained, when the cross-over point 23 is as close to electrode 6i as possible, without being in aperture 65 or beyond. In the tube described, the spacing between electrodes 56 and 60 is close to 31 mils.

The above described arrangement of electrodes 56, 60 and 64 maintains a cross-over region at a high uniform potential, instead of at intermediate potential between that of cathode 56 and that of electrode 64, as would be true without the pres.. ence of electrode 60. During tube operation the potentials of electrodes 56 and 60 are maintained at constant values, and will not undergo any potential changes which would tend to alter the position of the cross-over point.

The cross-over point 23 is never a single point on the gun axis. Since the electrons emitted from the cathode surface 'i8 leave at different velocities and pass through non-uniform portions of the focusing electrostatic field between the cathode surface 78 and electrode 60, there will be produced an aberration or a spreading of the focus point 23 along the gun axis. Point 23 represents more correctly a region of electron cross-overY points. Since this crossover region of the electron beam occurs in the field free space between electrodes 60 and 64, not only is the crossover formed at a high potential to give a better focus of beam 22 on target I8, but all portions of the electron beam will form a crossover at the same potential which provides a more uniform focus of the beam on target I8.

The electron gun structure as disclosed in Figure 2 has proved to be of considerable advantage in television pickup tubes, which utilize a low velocity scanning beam. The particular structure, described in detail above, provides an electron beam, which has a small range of electron velocities, and one of small cross-sectional area. The portion of the electron beam 22 having large transverse velocities are eliminated by the control electrode 56 closely spaced from the cathode emitting surface 'I8 and maintained at a large constant negative potential during tube operation, by providing a small eiective electron emitting cathode area, and by utilizing a small masking aperture to eliminate the components of the electron beam diverging from the cross-over point. Furthermore, the electron beam is maintained concentrated and of minimum cross-section by the small emitting cathode area, and by the action of accelerating lens 60 to accelerate the electrons to their greatest velocity before cross-over, which reduces the tendency of beam divergence from mutual repulsion of the electron charges. By forming the cross-over of the electron beam in a field-free space, between electrodes 60 and 64, there results less aberration and divergence of the beam focus at the target IB.

NYM,

Allof thls results ina concentratedelectron scanning beam ci small cross-section having a small cross-over point producing -a sharper focus at the target. My new gun structure vgives better alignment of Vthe electron beam components, produced by a greater packing of the electrons in the beam. Less 'dispersion oi the electron beam gives a better picture resolution through a more euicient discharge of the target.

While certain specific embodiments have been illustrated Yand described, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

1. AnV elec-tron gun structure comprising, a cathode electrode having an activated `surface `for providing a source of electrons, an aperture'd accelerating electrode adapted to be positively biased relative to said cathode electrode and 'f spaced from said cathode electrode, an apertured control electrode adapted to be biased at a constant negative potential relative to said control electrode and positioned in spaced and facetoface relationship to said accelerating electrode between said accelerating electrode and said cathode electrode, an apertured masking electrode positioned on the other side of said accelerating electrode, the apertures of said electrodes and said activated cathode surface arf ranged inV alignment to provide a straight line path for said electrons, means electrically connecti'rrg v,together said accelerating and masking electrodesto form a field free space therebetween,

said accelerating electrode and control electrode both positioned and arranged relative to' said cathode electrode to form the electrons from said cathode electrode into 'an electron beam with a crossover point in said eld 4free space, the aperture of said masking electrode being smaller than the apertures in said control and accelerating electrodes to permit only the passage of a center portion of said electron beam after its divergence from said crossover point, the aperturenofsaid control lelectrode being smaller than the activated surface of said cathode and smaller than the aperture of said accelerating electrode.

2. An electron discharge device comprising, an envelope, a cathode electrode within said envelope adapted to be connected to a source of electrical potential for providing an electron emission, a masking electrode spaced in said enelope from said cathode electrode, a control electrode positioned in said envelope between said cathode and said masking electrode, and an accelerating electrode in said envelope in spaced and ace-to-face relationship between said control and masking electrode, said control, masking and accelerating electrodes each having an aperture therethrough, said apertures being in alignment to provide rectilinear passage for said electron emission from said cathode electrode, means connected to said control electrode for connecting said control electrode to a source of constant negative potential relative to said cathode potential, means for connecting said accelerating and masking electrodes to a common source of constant positive potential relative to said cathode potential to provide a fieldeiree space therebetween, said accelerating electrode being positioned relatively close to said negative control electrode to provide with said control electrode an electrostatic eld for forming said electron emission into an electron beam having a crossover point in said field-free space during tube operation, the

I aperture vthrough'said accelerating electrode Vbeing larger than the aperture through said control n electrode.

3. An electron discharge device comprising, electrode means including a cathode for directing electrons as a beam valong a predetermined path, a target in the path of said beam adapted to have charges established thereon, means adja. c'ent for establishing a magnetic field axially of said beam path, said cathode having an electron emitting surface, a rst electrode positioned closely adjacent said cathode surface and having a single aperture registering with a central portion of said emitting, surface and adapted to have a negative potential applied thereto, a second electrode spaced from and in face-tomface relationship to Vsaid iirst electrode and 'having an aperture aligned with the aperture in said first electrode, the aperture of said second electrode being of larger diameter than the aperture'of said rst electrode, and a third electrode positioned adjacent said second electrode and having an aperture coaxial with the apertures of said rst and second electrodes but smaller than either ofthe apertures thereof, lead means connected to said second and third electrodes to provide a common positive potential thereto to form the electron beam from said 'cathode into a crossover between said last two apertured electrodes.

4. An electron discharge device comprising, an elongated envelope, a target at one end of said envelope and an electron 'gun assembly at the other end oiV said envelope for directing a stream of electrons toward said target, said gun assembly including a tubular member closed at one end, anV elongated cathode having an emitting surface and an insulating member supporting said cath'- ode within said tubular member, a iirst electrode supported within said tubular member closely adjacent the emitting surface of said cathode, said rst electrode having a single aperture aligned with a small central portion of said emitting surface, a second electrode positioned in spaced and fa'ce-to-face relationship to said first electrode within said tubular member, s aid second electrode having a depressed portion positioned closely adjacent said iirst electrode, saidY depressed portion having an aperture aligned with the aperture in said rst electrode but of larger diameter, said tubular member having an aperture in its closed end coaxially aligned with the apertures of said rst and second electrodes and smaller than the other two apertures, and an insulating spacer member insulatingly supporting the rst electrode Within the tubular member, said second electrode being electrically and conductively connected with said tubular member.

5. An electron discharge device having an elongated envelope, a target positioned adjacent one end of said envelope, a photosensitive surface applied to the inside of said envelope adjacent said target for emitting photo electrons directed toward said target during operation of said device, an electron gun assembly positioned at the other end of said envelope for directing an electron beam toward the other side of said target, said gun assembly having means for generating a beam of electrons in which the majority of said electrons approach said target normally and including a cathode electrode having an emitting surface, a first apertured electrode spaced closely adjacent said cathode and having an aperture registering with the central portion of said emitting surface, a second electrode having a larger aperture registering with rst said aperture and a third electrode spaced from said second electrode and having an aperture coaxial with the apertures in said rst and second electrodes, lead means connected to said electrodes to maintain said first electrode at a negative potential with respect to said cathode electrode and to maintain said second and third electrodes at a common positive potential with respect to said cathode, and means surrounding said cathode for receiving returned electrons from said target.

6. A television transmitting tube comprising, an evacuated envelope, an electron gun assembly mounted within said envelope for providing an electron beam along a path, a target electrode mounted within said envelope transversely to said electron beam path, said electron gun assembly including a tubular supporting member and a cathode electrode within said tubular member, an apertured plate electrode positioned transversely within said tubular member and adjacent said cathode electrode, insulating means fixedly supporting said cathode and plate electrodes Within said tubular member, lead means connected to said apertured plate electrode to provide a negative potential thereto during tube operation for controlling electron emission from said cathode, an accelerating electrode within said tubular member and in electrical contact therewith, said accelerating electrode having an aperture aligned with the aperture of said plate electrode, lead means connected to said accelerating electrode to provide a positive potential thereto during tube operation for accelerating and focusing the electrons from said cathode, plate means intercepting the electron beam path and closing one end of z.:

emitting surface, a rst electrode positioned close ly adjacent said cathode surface and having a single aperture registering with a central portion of said emitting surface, lead means connected to said first electrode and said cathode electrode for biasing said first electrode to a constant negative potential relative to the potential of said emitting cathode surface, a second electrode spaced from and in face-to-face relationship to said rst electrode and having an aperture aligned with the aperture in said first electrode, the aperture of said second electrode being of larger diameter than the aperture of said first electrode, a third electrode positioned on the opposite side of said second electrode from said rst electrode, said third electrode having an aperture co-axial with the apertures of said rst and second electrodes but smaller than either of the apertures thereof, lead means connected to said second and third electrodes for positively' biasing said second and third electrodes to a common potential relative to said cathode surface to form the electrons from said cathodeemitting surface into a cross-over between said second and third apertured electrodes.

RALPH E. JOHNSON.

REFERENCES CITED The following references are of record in the le of this patent:

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